This dissertation deals with the Lewis acidic properties of several transition metal compounds, especially iron, ruthenium and gold compounds. The catalytic activity of compounds of the above mentioned metals in several reactions was systematically
investigated, the focus being on complexes with only weakly coordinating ligands attached to the metal centre. Thus, the Lewis acidity is greatly increased as compared to complexes with strongly coordinating ligands. The reactions investigated comprise
particularly Friedel-Crafts type reactions, the hydroarylation of alkynes, and Mukaiyama aldol reactions. Especially gold-catalyzed protocols for the hydroarylation of alkynes turned out to be very valuable. In this context, an extraordinary property of
gold Lewis acids became apparent, their carbophilic behaviour, which is a direct consequence of the very soft character of cationic gold complexes.